Elevator car doors in elevator systems must be closed during movement of the elevator car within the hoistway and while the elevator car is "parked" at a landing. Some elevator codes also require that a certain minimal force be applied to maintain the elevator car doors closed as the elevator car travels within the hoistway. Additionally, for performance reasons, it is desirable to prevent elevator car doors from rattling as a result of vibrations in the hoistway.
Typical elevator car door systems that use conventional rotary motors and mechanical linkages include a counterweight to maintain the elevator car doors closed while the elevator car is moving. Also, conventional rotary motors continue exerting a relatively small electrical force to maintain the elevator car doors closed.
Some modern elevator car doors eliminate mechanical linkages with the counterweight, and instead are driven by a linear motor. Placing a counterweight on elevator car doors driven by a linear motor is impractical for a number of reasons. First, an additional weight contributes to moving mass of the door system which is undesirable. Second, the counterweight requires additional hardware, thereby increasing the cost and reducing reliability. Furthermore, in linear motor driven door systems, the space is limited and the counterweight cannot be accommodated.
Another option for linear motor driven door systems is to maintain the elevator car doors in the closed position by a force generated by the linear motor. However, using a linear motor to maintain the elevator car doors in the hold closed position has a number of disadvantages. One disadvantage is that it may contribute to overheating the motor, thereby reducing the life of the motor. Another disadvantage to utilizing the linear motor to maintain elevator car doors in a hold closed position is the additional consumption of energy. Therefore, it is desirable to have an effective scheme for maintaining elevator car doors in a hold closed position.
In Japanese patent application 4-28690, published Jan. 31, 1992, an elevator door has main rollers settle into recessed portions of their tracks, so the entire door is lowered as it closes. On opening, however, the mass of the entire door must be moved upwardly to disengage the door rollers from the recesses, which consumes significant power, requiring a larger door motor, putting strain on the entire door operating system, and slowing down the door opening process.